Low-coherence interference measurement is a technique involving sufficient resolution to enable information such as the surface shape or thickness of a measurement target to be obtained at a scale almost equal to the wavelength of light. Tomographic information on the measurement target can be obtained using light with a wavelength at which light transmits the measurement target, and thus, low-coherence interference measurement is utilized, for example, for medical devices. An example of a medical device is an optical coherence tomography apparatus that acquires biological tomographic information. This medical device enables information other than biological tomographic information to be obtained by using a combination of low-coherence interference measurement and near-infrared spectroscopy (NIRS) or polarimetry.
Low-coherence interference measurement involves performing measurement by separating light emitted by a low-coherence light source into probe light and reference light, irradiating the measurement target with the probe light, and detecting the intensity of interference light between the reference light and the probe light reflected from the measurement target. An intensity change resulting from interference occurs when the difference in optical path length between the probe light and the reference light is equal to or smaller than the coherence length of the light source. An apparatus utilizing low-coherence interference measurement includes an optical delay apparatus that enables the optical path length of the reference light to be changed. For the optical delay apparatus, there has been a demand to enable a size reduction and an increase in optical path length.